To provide cushioning protection and support for the foot, the sole of an athletic shoe commonly has a multi-layer construction comprising an outsole, a midsole, and an insole. The outsole is normally formed of a resilient and durable material to resist wearing of the sole during use. In many cases, the outsole includes lugs, cleats or other elements to enhance the cushioning and traction afforded by the sole. The midsole ordinarily forms the middle layer of the sole and is typically composed of a soft foam material to attenuate and dampen impact energy and distribute pressures placed upon the foot during athletic activities. The midsole may be formed with or without the inclusion of other cushioning elements, such as resilient gas-filled bladders. An insole layer is usually a thin, padded member provided overtop of the midsole to enhance the comfort afforded to the wearer. The performance of such footwear depends in large part on the ability of the sole to effectively cushion applied loads relative to the anatomy and skeletal structure of the foot and to stabilize the movements associated with running and like activities.
As to be expected of the running population, there are many different running styles. Nevertheless, approximately eighty percent of the running population makes first contact with the ground along the rear lateral corner 53 of the sole when running at moderate speeds. Individuals having this running style are commonly referred to as rearfoot strikers. By and large, the remaining twenty percent of the running population generally makes first contact along the lateral side of the sole. Often, first ground contact for these runners will occur at or between proximal head 31e of fifth metatarsal 34e and metatarsal-phalangeal joint 35e.
It has been common practice to use a thin band about the entire perimeter of the sole or otherwise indiscriminately proximate to those portions of the rearfoot, midfoot or forefoot associated with sudden impact events. Shock waves generated by an impact travel at a rate exceeding 1500 meters/second in soft tissue, and at a rate exceeding 3000 meters/second in bone. Accordingly, a soft cushion is needed proximate the point of impact to provide a rapid and sufficient cushioning response. Some of the most potentially damaging accelerations associated with running occur during the first few milliseconds of footstrike; that is, long before the cushioning materials located more centrally in a shoe sole can be brought to bear upon the support surface.
As can be appreciated, the construction of the sole must be coordinated with the foot structure and running styles in order for a sole to provide effective cushioning and stability. The skeletal structure of foot 8 provides the requisite strength to support the weight of the body during the ground contact and the remainder of the ground support phase. The structure of the foot can be categorized into three areas--namely, rearfoot area 9a, midfoot area 9b, and forefoot area 9c (FIG. 4).
Rearfoot area 9a includes talus 13 and calcaneus 17. The tibia and fibula of the leg are movably attached to talus 13 to form the ankle joint. In general, these leg bones form a mortise into which a portion of talus 13 is received to form a hinge-type joint which allows both dorsi and plantar flexion of the foot. Talus 13 overlies and is movably interconnected to calcaneus 17 to form the subtalar joint. The subtalar joint enables the foot to move in a generally rotative, side-to-side motion. Rearfoot pronation and supination of the foot is generally defined by movement about this joint.
Midfoot area 9b is anterior to rearfoot area 9a and comprises navicular 20, cuboid 21 and outer, middle and inner cuneiforms 22-24. The four latter bones 21-24 facilitate interconnection of the tarsus to the metatarsus.
Forefoot area 9a is anterior to midfoot area 9b and includes: metatarsals 34a, 34b, 34c, 34d, 34e; metatarsal-phalangeal joints 35a, 35b, 35c, 35d, 35e; sesamoids 36a, 36b; proximal phalanges 45a, 45b, 45c, 45d, 45e; distal phalanges 46a, 46b, 46c, 46d, 46e; and middle phalanges 47b, 47c, 47d, 47e.
Each metatarsal 34a-e is aligned with and attached via connective tissue to one of the proximal phalanges 45a-e at metatarsal-phalangeal joints 35a-e. For example, first metatarsal 34a is connected to proximal phalange 45a of the big toe 40a, and fifth metatarsal 34e is connected to proximal phalange 45e of the smallest or fifth toe 40e. First, second and third metatarsals 34a-c are largely attached on their proximal ends to outer, middle and inner cuneiforms 22-24, respectively. Fourth and fifth metatarsals 34d-e are both substantially connected to cuboid 21. Toes 40a, 40b, 40c, 40d, 40e are hingedly attached to the metatarsals for significant movement.
During running, the ground support phase of a step generally includes a braking phase, a stance phase, and a propulsive phase. The braking phase occurs when the foot makes first contact with the ground and the foot begins to flatten. The stance phase follows the braking phase and is generally considered to consist of the time when the foot flattens and the runner's center of gravity is generally located above the foot. The propulsive phase is characterized by the rising of the heel from the ground and the shifting of the runner's weight to the ball and toes of the foot.
With respect to a rearfoot striker, the foot at heel strike is typically oriented with big toe 40a pointing upward and slightly outward (FIG. 5). From the moment heel strikes the ground, and through the braking and stance phases, the foot rotates inwardly (i.e., the foot everts or pronates) and toward the midline of the body (i.e., adducts). During the propulsive phase, the foot rotates outward (i.e., inverts or supinates) and away from the midline of the body (i.e., abducts).
For a rearfoot striker, the plantar center of pressure path 55a normally proceeds from the point of first contact 53 towards the midline of the foot and exits between the first and second toes 40a, 40b (FIG. 6). This action reflects the fact that the individual has maintained balance and stability during the ground support phase. While eversion of the foot in this context is a natural action, excessive eversion or an excessive rate of eversion is sometimes associated with injuries among runners and other athletes. A deviation of the center of pressure path to beneath the first toe 40a is indicative of excessive eversion.
For a midfoot striker, first contact with the ground 56 is commonly made near proximal head 31e of fifth metatarsal 34e (FIG. 7). With these runners, the plantar center of pressure path 55b generally moves to the midline of the foot and then exits between the first and second toes 40a, 40b. For a forefoot striker, first contact with the ground 57 is often proximate the fifth metatarsal-phalangeal joint 35e (FIG. 8). The center of plantar pressure path 55c for this type of runner often moves rearward towards the midline of the foot before moving forward and exiting between the first and second toes 40a, 40b.
Moreover, an individual characterized as a rearfoot striker when running at slow or moderate speeds will often modify their technique to become a midfoot striker, and subsequently, a forefoot striker when running at ever increasing speeds. Further, when an individual is in a full sprint as on the straightaway of a running track, the initial point of contact between the foot and the support surface 58 may be near the distal head 33b of the second metatarsal 34b (FIG. 9). The plantar center of pressure path 55d can then proceed directly anteriorly between the first and second toes 40a, 40b.
Up until about the 1970's, athletic shoes by and large lacked sufficient cushioning. Consequently, injuries were sustained by those engaging in athletic activities. To overcome these shortcomings, manufacturers focused their attention upon enhancing the cushioning provided by athletic shoes. To this end, midsoles have over time been increased in thickness. These endeavors have further led to the incorporation of other cushioning elements within the midsoles and other sole configurations intended to provide enhanced cushioning effects. The industry's focus on improving the cushioning effect has resulted in a marked improvement of shoes in this regard.
Athletic footwear has frequently incorporated a thin band of elastomeric material, such as rubber, around the perimeter of the sole. For example, the Converse All-Star basketball shoe has such a band around the entire perimeter of the shoe. This type of construction is illustrated in FIGS. 1 and 2, wherein a shoe 1 is formed with an upper 2 and sole 3. The sole includes an outsole 3a, a midsole 3b, and an insole 3c. The upper is affixed to a t-stock or board 4 with stitching 5. A thin band stabilizer 6 is affixed along the entire perimeter of the sole by an adhesive, autoclaving or other means. This band is commonly affixed to both the sole and the upper to reinforce the bond therebetween.
Alternatively, the band is applied only around the toe area in many running shoes, including the EB 1120 shoe by Brutting and Lydiard, and Adidas shoes SL72 and SL76. In addition, other shoes, such as the Adidas Marathon Trainer shoe, has the band applied around the toe and heel areas of the shoe. Similarly, many track and field shoes have included an outsole wrap on the lateral side of the shoe adjacent the fifth metatarsal-phalangeal joint for the purpose of preventing abrasion to the shoe upper and enhancing stability of the foot. Examples of these type of shoes include the Adidas Adistar Sprint and Tiger Spartan B. Other shoes having different kinds of stabilizers, known as midsole wraps, have also been used. For instance, U.S. Pat. Nos. 4,259,792 to Halberstadt and 4,322,895 to Hockerson disclose midsoles extending about the lasting margin of the sole with the shoe upper and encompassing a portion of the shoe upper. Also, stabilizing devices such as taught in U.S. Pat. No. 5,046,267 have been used to increase stiffness in compression exhibited by the medial rearfoot area of the shoe sole. In any event, application of a stabilizing band has in the past been applied indiscriminately with respect to concerns for cushioning.
The band, before being applied to the shoe, is generally relatively elastic and flexible. However, it has not been generally recognized that subsequently affixing this same thin band to the side of the sole and the upper can significantly increase the local stiffness in compression exhibited by the shoe sole. As long as the bond between the thin band and the shoe remains intact and the thin band is not permitted to fold or buckle, compression of the sole portion also requires that the thin band be simultaneously compressed along its transverse or shear plane. The result is a sole which has a much greater stiffness in compression than would be experienced without the band.
As an example, two shoes were tested for relative stiffness in compression by an Instron device Series IX Automated Materials Testing System 1.15. The two shoes were the same, except for a thin band that was applied to the perimeter one of the shoes. The shoe without a thin band exhibited along its edge a stiffness in compression of 9.4 kg/mm. On the other hand, the shoe provided with the band exhibited along its edge a stiffness in compression of 13.2 kg/mm. Accordingly, an increase in stiffness of nearly 30% is realized by application of the band. A graphic representation of the results of this test showing the stiffness in compression versus 0-5 mm of deflection are set forth in FIG. 3.
As opposed to shoes (such as the Converse All-Star Shoe) which provide a stabilizer about the entire perimeter of the shoe, the shoe 75 disclosed in U.S. Pat. No. 3,793,750 to Bowerman, includes a sole which provides enhanced cushioning around the entire perimeter of the sole (FIGS. 10-11). More specifically, shoe 75 has an upper 76 and a sole 77. Sole 77 includes a foam midsole 78 and an outsole 79 with lugs 80. When a load is applied to the sole, the lugs along the perimeter can deflect upwardly more readily, and thus provide greater cushioning, than lugs in the central region of the sole. The lugs along the perimeter experience less resistance because the lugs are bordered by foam material on only three of its four sides.
To illustrate the differences in the compression stiffness of the lugs, a shoe was tested with an above-described Instron device. The lugs of the shoe had a surface area of approximately 10 square mm and a height of approximately 6 mm. The lugs in the center of the sole had a stiffness in compression of 13.96 kg/mm, whereas the lugs along the perimeter had a stiffness of 9.33 kg/mm. As can be appreciated, this is a difference in stiffness of nearly 35%. FIG. 12 provides a graphic representation of the stiffness in compression versus millimeters of deflection for the lugs along the perimeter and in the middle of the sole.
While cushioning advances are important, the benefits realized in cushioning have sometimes led to a degradation of a shoe's stability. Inadequate running stability, like inadequate cushioning, can result in an increased risk of injury. As discussed above, undue amounts of eversion or inversion, or excessive rates of eversion or inversion, can lead to injuries for runners. Moreover, forces generated between the foot and the ground during the ground support phase can also produce visible and generally equal and opposite physical reactions in the lower extremities during the subsequent flight phase. For example, severe inward or outward rotation of the foot during the propulsive phase can produce rotative or counter-rotative movements in the lower extremities during the flight phase. These generally inefficient movements, commonly called "whips" by athletic coaches, can also be associated with an increased potential for injury.
In an effort to provide greater stability, some shoe soles have in the past included stabilizers about the periphery of the sole. For example, stabilizers in the form of stiffer cushioning materials have been provided in an effort to prevent excessive eversion. U.S. Pat. No. 4,551,930 to Graham et al. discloses a shoe wherein stiffer foam material is positioned about the entire perimeter of the sole. Similarly, U.S. Pat. No. 4,128,950 to Bowerman et al. discloses a shoe wherein stiffer foam material is positioned about the perimeter of the heel area. While the stability of these shoes in certain areas, like the shoes discussed above with the bands, can be enhanced, the stability is sometimes gained at the expense of cushioning. The introduction of materials having a greater relative stiffness in compression about the heel, in particular the lateral rear corner of the sole reduces the potential cushioning available during heel strike. Further, the use of stiffer material in the rear lateral corner can effectively create an extended lever arm which can actually compromise rearfoot stability by increasing both the rate and amount of exhibited pronation.